This invention relates to a process for making a light waveguide telecommunication cable in which the light waveguides are incorporated into ribbons which are housed in one or more slots. If the slots are formed in the periphery of a rod, such cables are slotted core ribbon cables. However, the method may also be practiced in cables having slots formed by other carriers, such as those having a "U" shape in cross-section.
Light waveguide cables are now commonly used to transmit information over long distances. Such cables have been used for years by providers of telephone service, and increasingly are coming into use in cable television networks. The large bandwidth, light weight, and proven long term reliability of light waveguide cables make them ideal for a variety of uses in the telecommunication industry.
An early example of a slotted core ribbon cable is described in German Patent No. 25 07 583, granted in 1984 and assigned to Siemens AG. This cable comprises a rod having a central strength member formed of nickel alloyed steel wires. About the central member is provided an extruded plastic layer having a plurality of outwardly opening peripheral slots. A light waveguide ribbon is inserted into each slot. The light waveguide ribbons are thus housed in chambers formed by the slotted rod and an overlying covering. There may be a metal armoring layer around the covering. Finally, the assembly is covered by an outer sheath of extruded plastic material.
The light waveguide ribbons themselves comprise a plurality of individually coated light waveguides which may be embedded in side-by side relation within a relatively flat common coating of plastic material. The common coating is sometimes called a matrix coating. Alternatively, the light waveguides may be sandwiched between two flat strips, each of which may be formed of plastic material. Each of the individually coated light waveguides commonly includes a colored layer to enable them to be identified and distinguished from each other. The coated light waveguides may be arranged in contacting relation, or may instead be arranged such that each is separated from adjacent light waveguides in the ribbon by the common coating material. Light waveguide ribbons typically utilize ultraviolet light cured acrylate material for the common coating as well as the coatings for the individual light waveguides.
It is now common for slotted core ribbon cables to include a stack of light waveguide ribbons in each peripheral slot. Each slot is formed so as to have a floor and two side walls. During stranding, the stack of light waveguide ribbons comprises a plurality of light waveguide ribbons, with a bottom ribbon lying along the slot floor in the radially innermost position and each subsequent ribbon placed over the underlying ribbon. In this fashion, each ribbon is arranged to be separated from the slot floor by the underlying ribbons in the stack, and the outermost ribbon in the stack is at the radially outermost position with respect to the axis of the rod.
The slots housing the light waveguide ribbons commonly are formed so as to proceed in helical fashion to minimize stress placed on the ribbons when the cable is pulled or bent during installation, or when the cable expands or contracts in response to changes in temperature. In the alternative, the slots may have a lay which repeatedly changes direction (hand); sometimes this formation is called S-Z, with reference to the shapes of the letters S and Z.
If it is desirable to strand the ribbons into the rod slots such that the ribbons have excess length at room temperature, then stranding may take place with the rod being under a relatively high strain and the ribbons being under relatively low strains. When tension on the core is released, the rod shortens, so that the ribbons become longer than the helical lengths of the slots.
Japanese patent application 60-239059, published May 7, 1987 as publication 62-98313, describes a slotted core cable having ribbon supply tensions of 400 g, 330 g, 250 g, 170 g, and 80 g respectively moving from the innermost ribbon to the outermost ribbon of a stack in a helical slot. Thus, the supply tension decreases from the innermost ribbon of the stack to the outermost ribbon of the stack.
Similarly, U.S. Pat. No. 5,193,134 describes slotted core cables in which the longitudinal tensions applied to the stacked ribbons in a helical slot decreases starting from the slot floor. In a first example, the longitudinal tensions applied to ribbons which are radially superposed are 330 g, 300 g, 240 g and 210 g, and in the second example are 220 g, 200 g, 180 g, 160 g and 140 g proceeding outward from the slot floor.
When a ribbon is stranded helically, each fiber in the ribbon is in a helical configuration. If P is the pitch (lay length) of the helix of a representative fiber in the ribbon and r is the radius of the helix, then the length L of the helical path over a pitch is given by the well known formula: EQU L.sup.2 =(2.pi.r).sup.2 +P.sup.2. (Eq. 1)
By taking differentials of both sides of Eq. 1, and noting that L and P are approximately equal when r is much smaller than P, then on replacing the differentials with incremental quantities one obtains: EQU .DELTA.L/L.apprxeq.(2.pi./P).sup.2 r.DELTA.R+.DELTA.P/P (Eq. 2)
As the core tension is released after stranding with the rod under a higher strain than the ribbons, the rod strain falls by the amount .DELTA.P/P. As a result, so long as tension remains in all of the ribbons, the strains in all the ribbons in a stack fall by the same amount, .DELTA.L/L, which equals .DELTA.P/P; there is no tendency for the ribbons to move radially (.DELTA.r=0). Considering the outermost ribbon, for example, with the lowest stranding strain in the stack when manufactured according to the prior art, when .DELTA.P/P falls by an amount numerically equal to the stranding strain of the outermost ribbon, then the strain on the outermost ribbon vanishes. The point of this zero ribbon strain is dependent on factors such as temperature and the particular stranding tensions placed on the ribbons and the rod. If one now uses Eq. 2 to describe the incremental changes beyond the point at which the strain in the outermost ribbon vanishes, the left hand side of the equation becomes zero because the outermost ribbon is no longer changing in length. Thus, the equation may then be written: EQU .DELTA.r/r.apprxeq.-(.DELTA.P/P)(P/2.pi.r).sup.2. (Eq. 3)
Because .DELTA.P is negative as the rod shortens, the right-hand side of Eq. 3 is positive, so the ribbon moves incrementally away from the axis of the rod. Finally, because all of the underlying ribbons were stranded at higher tensions than the top ribbon, the top ribbon begins to move in the radially outward direction away from the stack before the tensions vanish in any of the underlying ribbons. This process continues, with the ribbons lifting one at a time.
The stack may reform when all the ribbons have lifted. In the meantime, however, whenever adjacent ribbons have become separated, inter-ribbon friction is no longer present. Any mechanical perturbation of the ribbons may cause the stack to lose its integrity, no longer having a rectangular cross-section but instead having ribbons sticking out on one or both sides of the stack after it reforms. Space permitting, a ribbon in the slot may even turn and become lodged transversely to the rest of the ribbons.
Similar behavior may occur during temperature cycling, for example, during cooling from high temperatures when the core shrinks.
Loss of stack integrity can lead to impermissibly high attenuations in the light waveguides, harming the transmission capability of the cable.